1. Field of the Invention
The present invention relates to a fuel property determination apparatus for an internal combustion engine.
2. Description of Related Art
It is known that the properties of the fuel that is supplied to an internal combustion engine has an influence on the control of such things as the air-fuel ratio in the internal combustion engine. For example, because a heavy fuel has a lower volatility than a regular fuel, any control that is designed for a regular fuel may adversely affect the operability and/or exhaust emission of the internal combustion engine if performed when a heavy fuel is being used. Various attempts have been made to determine fuel properties so that control of an internal combustion engine suitable for the fuel properties can be achieved.
Japanese Patent No. 3863362 discloses an internal combustion engine that is equipped with ignition timing control means that performs feedback control of the ignition timing so that the engine speed of the internal combustion engine can converge to a target speed during idling after start-up. It is disclosed in Japanese Patent No. 3863362 that an ignition timing correction amount at a time when the engine speed is stabilized with respect to a target speed and an ignition timing correction amount at which the engine speed converges after the stabilization are calculated and fuel properties are determined based on the difference between the ignition timing correction amounts.
It is also known that a more accurate determination of fuel properties is made based on the ratio of the correction amount from a basic ignition timing corresponding to a target speed (which may be hereinafter referred to as “ignition timing correction amount”) to the allowable range of the ignition timing that is determined depending on the target speed, temperature and so on of the internal combustion engine (which may be hereinafter referred to as “maximum ignition correction range”).
Japanese Patent No. 4356079 discloses a fuel property determination apparatus that determines fuel properties based on a combustion parameter that is calculated based on the pressure in a cylinder of an internal combustion engine (the length of combustion period). It is disclosed in Japanese Patent No. 4356079 that the threshold value that is used to make a determination of fuel properties is set based on the temperature of the internal combustion engine to make a more accurate determination of fuel properties.
As described above, various attempts have been made to determine fuel properties so that control of an internal combustion engine suitable for the fuel properties can be achieved. It is also known that a more accurate determination of fuel properties is made based on the ratio of the ignition timing correction amount to the maximum ignition correction range. When control of air-fuel ratio and so on is performed based on fuel properties in this way, the operability and/or exhaust emission of the internal combustion engine can be maintained in a favorable condition.